Field
This case relates to nuclear magnetic resonance (NMR) imaging methods. More particularly, this case relates to NMR imaging methods that provide, among other things, an ability to resolve short “T2” components. This case has applicability to the imaging of rocks, including rocks previously or presently bearing hydrocarbons, although it is not limited thereto.
Description of Related Art
Nuclear magnetic resonance (NMR) involves the application of a magnetic field to an object that impacts the magnetic moment (spin) of an atom in the object. In general, the magnetic field causes the atoms in the object to align along and oscillate (precess) about the axis of the applied magnetic field. The spin of the atoms can be measured. Of particular interest is the return to equilibrium of this magnetization; i.e., relaxation. For example, a state of non-equilibrium occurs after the magnetic field is released and the atoms begin to relax from their forced alignment. Longitudinal relaxation due to energy exchange between the spins of the atoms and the surrounding lattice (spin-lattice relaxation) is usually denoted by a time T1 when the longitudinal magnetization has returned to a predetermined percentage (i.e., 63%) of its final value. Longitudinal relaxation involves the component of the spin parallel or anti-parallel to the direction of the magnetic field. Transverse relaxation that results from spins getting out of phase is usually denoted by time T2 when the transverse magnetization has lost a predetermined percentage (i.e., 63%) of its original value. The transverse relaxation involves the components of the spin oriented orthogonal to the axis of the applied magnetic field. The T2 measurement is often performed using a well-established Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence which utilizes an initial 90 degree excitation pulse followed by a series of 180 degree (pi) refocusing pulses, and the data is typically analyzed using a Laplace inversion technique or an exponential curve fit.
NMR relaxation such as measured by T2 has been shown to be directly proportional to the surface-to-volume ratio of a porous material,1/T2=ρ·S/Vp  (1)    where S is the total surface area of the material,    Vp is the pore volume, and    ρ is the surface relaxivity.Surface relaxivity ρ is a quantity (in micron/second) that defines the strength of the surface relaxation phenomenon. Because of this relationship, NMR is extensively used in petroleum exploration to obtain estimates of porosity, pore size, bound fluids, permeability, and other rock and fluid properties (i.e., “petrophysical data”). For example, it is known that the T2 distribution is closely related to the pore size distribution. Reservoir rocks often exhibit a wide range of T2 due to the difference in pore sizes, with observed T2s from several seconds down to tens of microseconds. Typically, signals at long T2 (e.g. >100 milliseconds) are from large pores and such fluids are considered to be producible. For shorter T2 signals, 3-50 milliseconds, the fluids are often considered to be bound by capillary force of the pores. For example, in sandstone rocks, signals at T2 below 30 ms are considered bound by capillary force and will not produce. Thus, a cutoff value, T2cut, e.g., T2cut=30 ms can be used to calculate the bound fluid volume
                    BFV        =                              ∫                          T              ⁢                                                          ⁢              2              ⁢              m              ⁢                                                          ⁢              i              ⁢                                                          ⁢              n                                      T              ⁢                                                          ⁢              2              ⁢              cut                                ⁢                                    f              ⁡                              (                                  T                  ⁢                                                                          ⁢                  2                                )                                      ⁢            d            ⁢                                                  ⁢            T            ⁢                                                  ⁢            2                                              (        2        )            where f(T2) is the T2 distribution, and    T2min is the minimum T2 obtained in the T2 distribution.    If f(T2) is the T2 distribution for the fully saturated sample, then the porosity Ø can be obtained by integrating f(T2) according to
                    ∅        =                              ∫                          T              ⁢                                                          ⁢              2              ⁢              m              ⁢                                                          ⁢              i              ⁢                                                          ⁢              n                                      T              ⁢                                                          ⁢              2              ⁢              m              ⁢                                                          ⁢              ax                                ⁢                                    f              ⁡                              (                                  T                  ⁢                                                                          ⁢                  2                                )                                      ⁢            d            ⁢                                                  ⁢            T            ⁢                                                  ⁢            2                                              (        3        )            where T2max is the maximum T2 exhibited in the sample. Signals with even shorter T2, such as T2<3 milliseconds, are often due to clay bound water or viscous (heavy) hydrocarbon. Some rocks contain a significant amount of kerogen that is solid organic matter and which may exhibit T2s down to tens of microseconds.
Conventional magnetic resonance imaging (MRI) techniques that work well for long T2 signals fail for short T2 signals. In particular, conventional methods such as the Multiple-Slice-Multiple-Echo (MSME) imaging technique use slice selection (discussed below), frequency encoding and phase encoding. Both frequency and phase encoding require that the gradient pulses be switched on and off between each of the adjacent refocusing pulses (pi pulses). Gradients for slice selection must also be turned on and off for each refocusing pulse as they will interfere with the frequency encoding pulses. Each switching procedure typically takes several hundred microseconds. As a result, the minimum echo time that can be achieved by the frequency encoding and phase encoding techniques is generally on the order of several milliseconds, preventing the resolution of shorter T2 values. “Lengthy” echo times (on the order of several milliseconds) also pose the problem that in order to obtain a sufficient signal to noise ratio (SNR) required to resolve each of the image elements (˜1 mm3), relatively higher magnetic fields are necessary. However, with rock samples, at higher fields, a competing source of decay due to diffusion of the fluid and the induced magnetization of the rock will dominate and artificially shorten the apparent T2. The lengthy echo time of conventional MRI worsens the effect and further limits the samples appropriate for analysis.
Slice selection refers to the use of the differences in frequency response of the spins to a particular radio frequency (RF) pulse in the presence of an inhomogeneous magnetic field, and is a common component of MRI imaging. Typically, as in MSME, this is done to isolate a slice in the sample for imaging the sample with other image encoding techniques, i.e. phase encoding and frequency encoding. A gradient pulse will generate an approximately linear ramp in magnetic field strength that changes along a chosen direction in space. Because the frequency of the spins is proportional to field strength, the spin frequency will also form a linear ramp across the sample. As an RF pulse of finite duration and power will interact with spin of a limited range of frequencies, in the presence of a gradient this will interact with spins in a limited region of the sample and hence an MRI sequence will only image this portion of the sample. As the shape of the amplitude profile, the length, and frequency of the RF pulse will determine the exact nature of the response of spins at different frequencies and the amplitude and direction of the applied gradient can be controlled, the position and width of the slice can determined. Furthermore, the profile of excitation within the slice (as in Hadamard imaging) can also be controlled for further resolution as a function of slice depth. However, these techniques are combined with other image encoding methods (i.e. frequency encoding, phase encoding).